Carbonates such as ethylene carbonate, propylene carbonate and dimethyl carbonate are used as a solvent for an electrolyte salt of lithium secondary battery, solar cell, radical battery and capacitor. However since a flash point is low and combustibility is high, there is a danger of firing and explosion attributable to over-charging and over-heating, and in addition, since viscosity is high and conductivity at low temperatures is low, there is a problem that an output is decreased.
In addition, in the case of lithium secondary batteries, improvement in a withstand voltage of an electrolytic solution is demanded for increasing a capacity. Further in the case of capacitors, it is desirable that both of a negative electrode and a positive electrode are made of hard carbon, particularly it is desirable that a capacitor can be used stably at a voltage of not less than 3 V. In the case of solvents for electrolyte salts such as propylene carbonate and dimethyl carbonate which have been so far used, decomposition of an electrolytic solution occurs at a voltage of not less than 3 V, and therefore such solvents cannot be used.
To solve the mentioned problem, methods of adding a fluorine-containing ether have been proposed (JP6-176768A, JP8-37024A, JP11-307123A, JP2000-294281A, etc.).
However fluorine-containing ethers have lower viscosity at low temperatures as compared with carbonates having a fluorine-containing alkyl group at an end thereof, but there are disadvantages that solubility of a lithium salt which is an electrolyte salt is low, and since compatibility with hydrocarbon carbonates which are used as a solvent for dissolving an electrolyte salt is low, separation into two phases occurs. In addition, since a flash point is relatively low, improvement in flame retardance is necessary. Those are important problems to be solved.
In addition, use of a fluorine-containing chain carbonate which is obtained by fluorinating a chain carbonate is proposed (JP6-219992A, JP10-149840A, JP2001-256983A, JP2000-327634A, etc.).
However the carbonates in which an end of a fluorine-containing alkyl group is perfluoro (JP10-149840A and JP2001-256983A) have disadvantages that solubility of an electrolyte salt is low and compatibility with a hydrocarbon solvent is poor. In the case of carbonates having a hydrocarbon alkyl group at one end (JP6-219992A and JP2000-327634A), solubility of an electrolyte salt and compatibility with a hydrocarbon solvent are improved, but there are lowering of flame retardance and decrease in a withstand voltage due to a decreased fluorine content.
It has been proposed to use a fluorine-containing cyclic carbonate as an electrolytic solution. For example, there are proposals to use compounds obtained by replacing a part of hydrogen atoms of ethylene carbonate with fluorine atoms (JP5-325985A, JP10-189043A, JP2001-313075A, JP2003-168480A, etc.) and proposals to use compounds obtained by replacing a part or the whole of hydrogen atoms of methyl groups of propylene carbonate with fluorine atoms (JP8-37025A, JP10-233345A, etc.).
However with respect to the compounds obtained by replacing a part of hydrogen atoms of ethylene carbonate with fluorine atoms, synthesis and separation are difficult and flame retardance is not enough. Also with respect to the compounds obtained by replacing a part or the whole of hydrogen atoms of methyl groups of propylene carbonate with fluorine atoms, there are lowering of solubility of an electrolyte salt, lowering of a discharging efficiency and increase in viscosity, and thus the compounds do not always satisfy required performances. Further it is desirable that electrolytic solutions for lithium secondary batteries and capacitors are liquids at a temperature of not less than −20° C. However a trifluoromethyl cyclic carbonate is a solid at −20° C. and therefore there is a limit in use thereof for these applications.
In the case of electrolytic solutions for capacitors and radical batteries which repeat charging and discharging like lithium secondary batteries, improvement in flame retardance and a withstand voltage and enhancement of low temperature characteristics in which viscosity is not increased even at low temperatures and yet decrease in conductivity is small are desired.